Tolkien was employed as an editor of the OED for several years following his return from the First World War and the first fifth of The Ring of Words describes his work there, with details of the entries on which he worked, from "waggle" to "wold". It also touches on his ongoing connections with the OED, and on related work on a Middle English Vocabulary.
The second part of The Ring of Words looks at Tolkien as a wordwright. Etymological reconstructions require not only an understanding of phonological and morphological changes, but also the ability to imagine the social and semantic contexts in which words were used. So Tolkien's creation of new languages led to the invention of a history and mythology to accompany them, though his central interest remained philological. Tolkien's approach to the choice and creation of words shows the influence of precursors such as Walter Scott and William Morris and can be compared to that of contemporaries such as E.R. Eddison and C.S. Lewis. There is a shift in his writings from clunkier Morris-style archaisms to the more subtle style of The Lord of the Rings, which largely eschews overt archaism in favour of a more indirect use of historical background to produce elevated language.
The last three fifths of The Ring of Words consists of a hundred studies of particular words, looking at their histories and how Tolkien used — or in some cases created — them. These range in length from a paragraph on "backarapper" to ten pages on "hobbit". They look at new words such as "eucatastrophe" and "staggerment", revivals of old words such as "mathom" and "hame", reconstructions of how some Old English words might look in modern English if they had survived, and the derivations of proper names such as Marish and Quickbeam. A brief epilogue summarises Tolkien's influence on the English language.
The Ring of Words is perhaps a chance to "cash in" on the Tolkien boom, but it is not going to appeal to a really broad audience, only to those who share Tolkien's fascination with words and their history. It doesn't provide any general background, so readers who don't know anything at all about the OED, the relationship between Old and Middle English, and so forth are likely to find it a bit confusing. It also assumes readers are familiar with Tolkien's works, or at least with the major ones. Anyone with that background, however, and some curiosity about etymology and philology, should really enjoy The Ring of Words.